1. Field of the Invention
This invention relates to high temperature alloys for thermal equipment based on intermetallic compounds which are suitable for ordered solidification and to supplement the conventional nickel-based superalloys.
The invention relates to the further development and improvement of the alloys based on an intermetallic compound of the titanium aluminide TiAl type with further additives which increase the strength, the toughness and the ductility.
In the narrower sense, the invention relates to a high temperature alloy for machine components based on doped TiAl.
2. Discussion of Background
Gamma titanium aluminide is an intermetallic compound based on the formula TiAl. Notwithstanding its excellent oxidation resistance, high modulus of elasticity and low density, this intermetallic compound has not seen widespread industrial use in structural applications, due to the relatively low tensile ductility. In general, a minimum of 0.5% elongation is considered marginally acceptable for handling purposes during manufacturing and for actual service conditions.
Intermetallic compounds of titanium with aluminum have some valuable properties which make them appear attractive as structural materials in the medium and higher temperature range. These include, inter alia, their density, which is low compared with superalloys and reaches only about half the value for Ni superalloys. However, their brittleness stands in the way of their industrial applicability in the present form. The former can be improved by additives, in which case higher strength values may also be achieved. Possible intermetallic compounds, some of which have already been introduced, which are known as structural materials are, inter alia, nickel aluminides, nickel silicides and titanium aluminides.
Attempts have already been made to improve the properties of pure TiAl by slight modifications of the Ti/Al atomic ratio and by alloying with other elements. Further elements proposed were, for example, alternatively Cr, B, V, Si, Ta as well as (Ni+Si) and (Ni+Si+B), and also Mn, W, Mo, Nb, Hf. The intention was, on the one hand, to reduce the brittleness, that is to say to increase the ductility and toughness of the material, and, on the other hand, to achieve as high a strength as possible in the temperature range of interest between room temperature and operating temperature. An additional aim was a sufficiently high resistance to oxidation. These aims were, however, only partially achieved.
The high temperature strength of the known aluminides in the meantime still leaves something to be desired. Corresponding to the comparatively low melting point of these materials, the strength, in particular the creep resistance in the upper temperature range, is inadequate, as can also be seen from relevant publications.
U.S. Pat. No. 3,203,794 discloses a TiAl high temperature alloy containing 37% by weight of Al, 1% by weight of Zr and remainder Ti. The comparatively small addition of Zr causes this alloy to have properties comparable to those of pure TiAl.
EP-A1-0,365,598 discloses a high temperature alloy based on TiAl with Si and Nb additives, whereas in EP-A1-0,405,134 a high temperature alloy based on TiAl with Si and Cr additives is proposed.
A series of divisional patents of NAZMY et al., namely U.S. Pat. No. 5,342,577, a division of U.S. Pat. No. 5,286,443, a division of U.S. Pat. No. 5,207,982, discloses three types of doped titanium aluminide alloys. U.S. Pat. No. 5,207,982 focuses on titanium aluminide doped with 0.1-1.5 atom. % Si and 1-8 atom. % W, without B or Cr. U.S. Pat. No. 5,286,443 focuses on titanium aluminide doped with 0.1-1 atom. % B and 1-8 atom. % W and/or Cr, without Si. U.S. Pat. No. 5,342,577 focuses on titanium aluminide doped with 0.1-2 atom. % Ge and 1-4 atom. % W and/or Cr, without B or Si. U.S. Pat. No. 5,207,982 and U.S. Pat. No. 5,286,443 describe various Titanium Aluminide alloys. However, neither patent descibes any alloy combinations of boron and silicon in titanium aluminides modified by chromium and tungsten. It should be noted that the above ranges are calculated from the values recited in the NAZMY claims.
The following documents are also cited in respect of the prior art: N. S. Stoloff, "Ordered alloys-physical metallurgy and structural applications", International Metals Review, Vol. 29, No. 3, 1984, pp. 123-135. G. Sauthoff, "Intermetallische Phasen" ("Intermetallic Phases"), Werkstoffe zwischen Metall und Keramik, Magazin neue Werkstoffe 1/89, p. 15-19. Young-Won Kim, "Intermetallic Alloys based on Gamma Titanium Aluminide", JOM, July 1989, pp. 24-30. This prior art reference has recognized that the mechanical properties (tensile yield strength, ultimate tensile strength, and ductility) of TiAl are affected by deviations from Ti/Al stoichiometric ratio, and small additions of dopants to a non-stoichiometric TiAl composition, even in the range of 0.1 to 1.0 atomic percent. Also relevant is "Ordered Intermetallic Alloys, Part III: Gamma Titanium Aluminides", Young-Won Kim, JOM, July 1994, pp. 30-39.
Often, prior art dopants which are present as ternary additions to a non-stoichiometric TiAl composition impart unpredictable effects on strength or ductility, or both, as taught by U.S. Pat. No. 4,842,820. This patent further teaches that the nature and the concentration of the dopant, as well as the processing (annealing or heat treatment) temperature have a strong bearing on strength and ductility.
Other prior art has shown that, in some instances, although some dopants may produce beneficial effects when added singly, the presence of the same dopants in combination thereof can produce detrimental effects on strength and ductility. For example, U.S. Pat. No. 5,304,344 cites three example alloys, Ti.sub.49 Al.sub.48 V.sub.3, Ti.sub.50 Al.sub.46 Nb.sub.4 and Ti.sub.48 Al.sub.48 Ta.sub.4 (gamma alloy no. 14, 40, and 60 respectively of Table III) which exhibit room temperature ductility greater than 1.0%. However, when the additives vanadium, niobium and tantalum are combined in alloy Ti.sub.49 Al.sub.45 V.sub.2 Nb.sub.2 Ta.sub.2, a very low ductility (about 0.1%) results. Clearly, the prior art has conclusively demonstrated the unpredictable response to a combination of dopants that may be beneficial when added as ternary dopants to a non-stoichiometric gamma titanium alloy composition. Those skilled in the art will recognize that the situation is more complex when polynary additives are considered, as is done by the present inventor in this patent application.
It is useful to cite several key patents whose salient features constitute the prior art regarding the beneficial effect of boron as a doping agent in gamma titanium aluminides. Boron has been used to refine the grain size of metallurgical structures, its effectiveness to achieve such refinement being dependent on its concentration and the presence of other dopants in the gamma titanium aluminide composition.
U.S. Pat. No. 4,842,820 teaches the use of boron as a ternary dopant in concentrations varying from 1 to 5 atom. % to effectively achieve high strength and improved ductility.
In U.S. Pat. No. 5,080,860 boron is taught to be in concentrations from 0.5 to 2 atom. % in a gamma titanium aluminide composition containing niobium and chromium. The '860 patent shows chromium has no refining effect on the crystal form of the solidified structure as the aluminum content varies from 46 to 50 atom. %, with the crystal form changing from a large equiaxed structure to a columnar-equiaxed one. Further, the '860 patent prescribes the optimum boron concentration to be between 0.5 and 2 atom. % to achieve a fine grain equiaxed microstructure and property improvements. The same range of boron concentration is specified in U.S. Pat. No. 5,204,058 and U.S. Pat. No. 5,264,054, again for titanium aluminide compositions modified by niobium and chromium. On the other hand, in U.S. Pat. No. 5,205,875 the range of boron concentration varies from 0.1 to 0.2 atom. % for the titanium aluminide alloy Ti.sub.Bal. --Al.sub.46-48 --Cr.sub.2 --Nb.sub.2 --B.sub.0.1-0.2.
U.S. Pat. No. 5,082,624 and U.S. Pat. No. 5,082,506 relate to doping a niobium containing titanium aluminide with boron additive in concentrations between 0.5 and 2 atom. % in cast, and cast and thermomechanically worked ('506 patent) samples.
The use of boron as a quinary dopant in a titanium aluminide composition modified by chromium and tantalum is taught by U.S. Pat. No. 5,098,653, U.S. Pat. No. 5,131,959, U.S. Pat. No. 5,228,931 and U.S. Pat. No. 5,324,367. The specified ranges of boron concentration (in atom. %) vary as follows: 0.5 to 2 ('653 and '959 patents), 0.1 to 0.3 ('931 patent), and 0.05 to 0.2 ('367 patent).
Other relevant patents are U.S. Pat. No. 4,842,819, U.S. Pat. No. 4,842,820, U.S. Pat. No. 4,857,268, U.S. Pat. No. 4,836,983, and EP-A-0,275,391.
The properties of the known modified intermetallic compounds in general do not yet meet the technical demands for the production of usable workpieces therefrom. This applies in particular with regard to high-temperature strength and ductility. There is therefore a need for further development and improvement of such materials.
These and other difficulties experienced with the prior art alloys and processes have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the present invention to provide a low density alloy which has adequate resistance to oxidation and corrosion at high temperatures and at the same time a high-temperature strength and sufficient toughness in the temperature range of 500 to 1,000 degree(s) C., which alloy is very suitable for ordered solidification and essentially consists of a high melting point intermetallic compound.
It is a further object of the present invention to provide gamma titanium aluminide compositions containing boron, chromium, tungsten and silicon, which are particularly suitable for the manufacture of net-shape components by casting.
Additionally, it is an object of this invention to provide an alloy composition which exhibits adequate room temperature tensile ductility, i.e. minimum 0.5%, to allow handling and finishing of cast components without loss of structural integrity.
Further, it is another object of this invention to provide a gamma titanium aluminide composition which exhibits room temperature tensile strength higher than 75 ksi.
With the foregoing and other objects in view, which will appear as the description proceeds, the invention resides in the combination and arrangement of steps and the details of the composition hereinafter described and claimed, it being understood that changes in the precise embodiment of the invention herein disclosed may be made within the scope of what is claimed without departing from the spirit of the invention.